This invention relates generally to optical sensing of droplets in continuous stream type ink jet printers and more particularly to two-dimensional differential optical sensors for sensing the position of ink droplets relative thereto while they are in flight.
Generally, ink jet devices of the continuous stream type employ a printhead having multiple nozzles from which continuous streams of ink droplets are emitted and directed to a recording medium or a collecting gutter. The printhead has an aperture plate with at least one row of nozzles or orifices through which the ink is ejected under pressure to form a row of parallel streams. The ink is stimulated prior to or during its exiting from the nozzles so that the stream breaks up in a series of uniform droplets at a fixed distance from the nozzles. As the droplets are formed, they are selectively charged by the application of a charging voltage by electrodes positioned adjacent the streams at the location where they break up into the droplets. The droplets which are charged are deflected by an electric field either into a gutter for ink collection and reuse, or to a specific location on the recording medium, such as paper, which may be continuously transported at a relatively high speed across the paths of the droplets.
Printing information is transferred to the droplets through charging by the electrodes, the charging control voltages are applied to the charging electrodes at the same frequency as that which the droplets are generated. This permits each droplet to be individually charged so that it may be positioned at a distinct location different from all other droplets or sent to the gutter. Printing information cannot be transferred to the droplets properly, unless each charging electrode is activated in phase with the droplet formation at the associated ink stream. As the ink droplets proceed in flight towards the recording medium, they are passed through an electric field which deflects each individually charged droplet in accordance with its charge magnitude to specific pixel locations on the recording medium. Thus, to calibrate the ink jet printer so that the ink droplets impact the desired locations on the recording medium, the trajectories of the ink droplets must be determined and adjusted.
U.S. Pat. No. 4,510,504 to Tamai et al discloses an ink droplet sensor for a multi-jet ink jet printer comprising a light emitter and a plurality of light receivers. In one embodiment, each nozzle for ejecting ink droplets has a light emitter that is to one side of the ink droplet flight path or trajectory and a set of light receivers on the other side of the droplet trajectory. The light reflected from a passing droplet to a one of the receivers of the set determines, in combination with the intensity of the reflected light sensed, the flight path or trajectory of the droplet. In another embodiment, the light emitter and at least one of the plurality of receivers are aligned with the droplet trajectory of each nozzle. The light emitters and receivers are mounted in a common base plate and are substantially coplanar with each other.
U.S. Pat. No. 4,328,504 to Weber et al discloses an optical sensor which travels with the drop-on-demand type ink jet printhead as it traverses across the recording medium in a horizontal direction while the recording medium is moved or stepped in the vertical direction. The optical sensor observes the actual pixel location on the recording medium and circuitry compares it to the desired location. Corrective measures are energized as required in accordance with the comparison signal.
U.S. Pat. No. 4,255,754 to Crean et al discloses the use of paired photo-detectors to sense ink drops, one each for two output fibers that are used to generate an electrical zero crossing signal. The zero crossing signal is used to indicate alignment or misalignment of a droplet relative to the bisector of a distance between two output fibers. The sensor of this patent employs one input optical fiber and at least two output optical fibers. The free ends of the fibers are spaced a small distance from each other; the free end of the input fiber is one side of the flight path of the droplets and the free end of the output fibers are on the opposite side. The remote end of the input fiber is coupled to a light source, such as an infra-red light emitting diode (LED). The remote ends of each output fiber are coupled to separate photodetectors such as, for example, a photodiode responsive to infra-red radiation. The ink is substantially a dye dissolved in water and is, of course, transparent to infra-red light, thus reducing the problems of contamination usually associated with ink droplet sensors. The photodiodes are coupled to differential amplifiers so that the output of the amplifiers are measurements of the location of droplets relative to the bisector of the distance between the output fiber ends confronting their associated input fibers and droplets passing therebetween. Amplifier outputs are used in servo loops to position subsequently generated droplets to the bisector location. The zero crossing may be used, depending upon its orientation with respect to the droplet stream direction, as a time reference to measure the velocity of the drop. Therefore, the droplet velocity information may be used in a servo loop to achieve a desired velocity. The patent to Crean et al therefore discloses sensing the ink droplets in the plane of their travel and deflection.
Using an orthogonal coordinate system, the trajectory of the droplets from the nozzle to the recording medium is the Z axis, and the deflection of the droplets by the deflection field is the X axis. The direction of the droplet out of this XZ plane is the Y direction, and as disclosed in this patent, an ink droplet passing exactly through the bisector of the two output or receiving fibers cannot detect a misalignment in the Y direction. This is because each of the output fibers receive equal amounts of light from the input fiber regardless of whether the ink droplet is above or below the desired deflection plane of the ink droplets. By using one of these zero crossing signal detectors at a location between adjacent end most droplets thrown from separate adjacent nozzles, the stitch point between these droplets can be controlled so that the segments of each line of droplets to be printed by each nozzle may be adjusted to prevent gaps or overprinting on the recording medium. However, no sensing and control of the droplets above or below the desired deflection plane of the droplets concurrently with the sensing and control of the droplets in the XZ plane is possible with this configuration.
Therefore, a need remains for two-dimensional sensing of the ink droplets in order to control the misalignment of droplets above or below the deflection planes, as well as the misalignment of the droplets within such planes and such a sensor is the subject of the present invention.